1. Field of the Invention
The present invention relates to a method to determine a fiducial point, particularly to a method to determine a fiducial point for holographic data storage.
2. Description of the Related Art
The Holographic data storage device features both high speed and high capacity and thus has very high potential among next-generation data storage devices. However, the holographic data storage device demands very high quality in optical systems and system calibration because it is likely to be affected by noise (such as aberration) under the high transfer speed thereof. The abovementioned factors make the holographic data storage device hard to commercialize.
Refer to FIG. 1. A conventional holographic data storage device comprises a laser source 10, a spatial filter 12, a first lens 14, a beam splitter 16, a spatial modulator 18, a second lens 20, a recording medium 22, a third lens 24, a reflector 26, and a 2D detector 28. The beam emitted by the laser source 10 passes through the spatial filter 12, the first lens 14 and the beam splitter 16 and then is split into a reference beam 30 and a signal beam 32. The reference beam 30 proceeds toward the reflector 26. The signal beam 32 proceeds toward the spatial modulator 18. The reflector 26 reflects the reference beam 30 to the recording medium 22. The spatial modulator 18 encodes the signal beam 32. The encoded signal beam 32 also proceeds to the recording medium 22. The reference beam 30 interferes with the encoded signal beam 32 to form an interference pattern stored in the recording medium 22. In reading the data stored in the recording medium 22, the signal beam 32 is shielded, and the reference beam 30 is incident on the recording medium 22 to generate an interference pattern on the 2D detector 28. However, the image of the interference pattern may be distorted because of aberration, etc.
In order to recover the image received by the 2D detector, several image recovery technologies had been developed. For an example, a U.S. Pat. No. 5,982,513 uses the angle between the reference beam and the incident beam to calculate the location of the image. However, the prior art is unlikely to align the images that are out of focus. For another example, M. Ayres, et al. proposed a paper in 2006 “Image Oversampling for Page-Oriented Optical Data Storage”, wherein an appropriate proportion should be maintained between the 2D detector and the spatial modulator. Thus, the prior art cannot perform image processing for any system. For yet another example, R. M. Shelby, et al. proposed a paper in 1997 “Pixel-matched holographic data storage with megabit pages”, which involves a pixel-matched system demanding precise alignment, high quality optical systems and smaller aberration. Therefore, the prior art is too expensive to popularize. For a further example, M. Ou-Yang et al. proposed a paper in 2011 “A Gray Level Weighting Method to Reduce Optical Aberration Effect in Holographic Data Storage System”. In the prior art, a single threshold is corresponding to multiple regions in a block during binary conversion because of noise. Thus, the fiducial point is hard to determine in the prior art.
Accordingly, the present invention proposes a method to determine a fiducial point for holographic data storage to overcome the abovementioned problems.